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Penetrating the Fog

In the sky this week…

Saturn rises around 7 p.m.

The Moon will be New on May 2.The Moon will reach Last Quarter on April 24.

Ken Tapping, April 20, 2011

Have you ever noticed that when driving under dull foggy conditions, sunglasses with orange or brown lenses often make it easier for you to see? The glare from other people’s headlights is reduced, as is the glare coming back from your own car’s lights. The glare issue is even worse for those with blue headlights.

The cause of glare is a process called “scattering,” where light waves hit the tiny droplets of water that makeup the fog and scatter in all directions. The degree of scattering is enormously higher for short wavelengths of light than it is for longer wavelengths. Blue light has a shorter wavelength than red light and is much more severely scattered. Therefore, glasses that reduce the amount of highly-scattered blue light reaching your eyes, compared with the amount of much less-scattered red light, will make it easier to see.

Rho Ophiuchi is one of the closest star-forming complexes to Earth. The bright pink objects just left of center are young stellar objects – baby stars just beginning to form. In visible light, they are completely hidden in the dark nebula that surrounds them. Image Credit: NASA/JPL-Caltech/UCLA.

We find the same problem in astronomy. Many things we want to observe are buried within or behind great clouds of cosmic dust. For example, if you look southward on a summer evening, you are looking in the direction of the centre of our galaxy. However, thanks to the dust clouds, you are actually seeing only a small fraction of the distance to it. Once again, the problem is related to scattering. As in the case of the fog, we can get around the problem by observing at longer wavelengths, such as in the infrared part of the spectrum. Using cameras and imagers sensitive to infrared light, we can actually see the neighbourhood of the black hole at the centre of our galaxy. If we observe using radio telescopes, at far longer wavelengths than light, there is little if any sign of those dust clouds at all. We can see straight through.

Another area of great astronomical interest is the birth of new stars and planets. Since these bodies form from collapsing dust and gas clouds, the action is hidden in the densest part of the clouds. We only get to see light from the new stars after they have ignited and evaporated their birth clouds. Once again, the longer wavelengths provide a solution. By observing at infrared and sub-millimetre wavelengths, we can see inside and observe what’s happening.

We now have cameras and imagers that can deal with longer-wavelength, less scattered radiation, and telescopes designed for making observations in this part of the electromagnetic spectrum. Canada is a partner in the Gemini telescope project, which involves two telescopes with eight metre mirrors, one in the northern hemisphere (in Hawaii) and the other in the southern one (in Chile). Another instrument in which Canada has a share is the James Clerk Maxwell Telescope, which operates at millimetre and sub-millimetre wavelengths, the twilight zone between the infrared and radio wave parts of the spectrum. With instruments like these, we can be there at the birth of new stars and planets.

However, the film star telescope in which Canada is involved is ALMA, the Atacama Large Millimetre Array, which is under construction on the Atacama Plateau, in Chile. This imager, made up of many dish antennas, will show us things we have never seen before, in detail never before possible.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton, BC,
V2A 6J9.